1. Field of the Invention
The present invention relates to a storage drive such as a magnetic disk drive of a computer, and more particularly to a storage drive with improved tribological reliability even during a long running period, and to its data write method.
2. Description of the Related Art
In order to improve a record density of a recent storage medium, for example, a magnetic disk drive, the shorter a distance (generally called a flying height) between a head and a disk, the better. If the flying height is made low, retaining tribological reliability of a magnetic disk drive becomes an important issue.
Main causes of degrading the tribological reliability of a magnetic disk drive are damages of a head or disk by direct contact therebetween or indirect contact via contamination or the like in the storage drive. As the flying height lowers, a possibility of damages of the head or disk becomes larger as the time while the head flies over the disk becomes longer.
In order to improve the tribological reliability, a load/unload (hereinafter described as L/UL) mechanism has been used. This mechanism shortens the head flying time by retracting the head to an area outside of the disk surface (outside of the outer circumference of the disk) while the magnetic disk drive does not access the disk for read/write or the like. The L/UL mechanism is detailed, for example, in the document “Load/Unload Technology for Disk Drives” by T. R. Albrecht et. al., IEEE Transactions on Magnetics, Vol. 35, No. 2, pp. 857 to 862, March 1999, and in other documents. JP-A-9-17099 (IBM) also discloses technique of varying an unload timing in response to a plurality of commands.
A read cache function is known by which the number of accesses to a disk can be reduced. With read caching, data on the disk having a high access occurrence frequency is temporarily stored in a volatile memory, and when a read command of reading the same data is issued from an upper level device (host), the data is read directly from the volatile memory without accessing the disk.
A function similar to a read cache function is a write cache function for data write. With write caching, data to be written in response to a write command issued by a host is temporarily stored in a volatile memory. The magnetic disc drive notifies the host of a write completion when the data is written in the volatile memory (high speed buffer memory), and at a later timing writes the data on a low speed disk. In this manner, processing speed is increased. More specifically, with write caching, data to be written in response to a plurality of write commands time continuously issued from a host is temporarily stored in a buffer, and after all the write commands are received, all the data in the buffer is written in the disk sequentially.
Write caching technology using both a buffer memory and a non-volatile memory is disclosed in JP-A-7-44982. This technology pertains, however, only to backup of data during a power failure.